what's use of original_{0}" in update command in grid view

hi,
In grid view while giving update command what is the use of OldValuesParameterFormatString="original_{0}".Thou gh it is clear that this gives a format for some value but what is the significance of _{0} in "original_{0}" .It is giving error when i write _{1} instead of _{0} in "original_{0}".
so,why it is happening so?